Switching nanoprecipitates to resist hydrogen embrittlement in high-strength aluminum alloys

Hydrogen drastically embrittles high-strength aluminum alloys, which impedes efforts to develop ultrastrong components in the aerospace and transportation industries. Understanding and utilizing the interaction of hydrogen with core strengthening elements in aluminum alloys, particularly nanoprecipitates, are critical to break this bottleneck. Herein, we show that hydrogen embrittlement of aluminum alloys can be largely suppressed by switching nanoprecipitates from the η phase to the T phase without changing the overall chemical composition. The T phase strongly traps hydrogen and resists hydrogen-assisted crack growth, with a more than 60% reduction in the areal fractions of cracks. The T phase-induced reduction in the concentration of hydrogen at defects and interfaces, which facilitates crack growth, primarily contributes to the suppressed hydrogen embrittlement. Transforming precipitates into strong hydrogen traps is proven to be a potential mitigation strategy for hydrogen embrittlement in aluminum alloys.

In fact, I did not see any mention in the manuscript, but the AlMgZn(Cu) alloys, which is the metallurgical merge between 5xxx and 7xxx series of Al-based alloys, comprise an entire new class of alloys, which has been discovered just in the past 3 years: the aluminium crossover alloys. A recent review paper has been published on the topic and I suggest the authors to read it and possibly incorporate these ideas into their manuscript, if they find it appropriate and suitable (https://doi.org/10. 1016/j.pmatsci.2021.100873). Interestingly, one of the major features of novel aluminium crossover alloys is the precipitation of the Tphase -Mg32(Zn,Al)49 -which is a highly chemically-complex intermetallic phase and highlyconcentrated superstructure bearing hundreds of atoms in its unit cell; this is opposed to most conventional hardening precipitates (and not nanoparticles as the terminology used by the authors in their paper) in existing commercial aluminium alloys.
That said, I will now make some comments on the authors' paper: this is a very good piece of scientific research, but the excessive number of supplemental figures makes it almost impossible to follow its intended message by only reading the main manuscript text. I suggest the authors to re-evaluate the use of supplemental figures by incorporating the most relevant figures (i.e. those figures who are fundamental for major understandings) into the main paper. Comments on figure 1: this reviewer is not sure about the indexing of both eta-prime and T-phase using the SAED and FFT method along [110] zone axis. It is of my knowledge that these phases can be unequivocally distinguished along the [001] zone axis instead, please see these two references: https://doi.org/10.1016/j.actamat.2020.116617 https://doi.org/10. 1016/j.matdes.2020.108837 In addition, for an article of the Nature Communication's relevance, this reviewer finds it mandatory that the authors present some sort of chemical analysis of these precipitates: either APT or STEM-EDX. These phases can be very similar on a chemical nature standpoint, and unequivocal confirmations of their existence are mandatory for validate the main message of the paper, which is the hypothesis that T-phase precipitates contribute to a significant reduction in the concentration of hydrogen within an Albased alloy (a new crossover alloy type synthesized by the authors?). In some aluminium crossover alloys, the diffraction signal of T+eta-prime phases come together, but there are ways to distinguish them; I advise the authors to consult appropriate literature. Now a comment on semantics. Can a nanoparticle be considered a precipitate and vice-versa? Well, precipitates (i.e. a secondary phase that separate/partition out the alloy's matrix whilst still embedded to it) via natural and artificial aging are known to occur in aluminium alloys since 1938 (Nature 142, 569-570 and Nature 142, 570-570 both from the year of 1938!). The term "nanoparticles" gained only recent attention due to the emergence of nanotechnology as a consolidated field of research. Personal opinions in paper revisions can cause stress to the authors, but it is the subjective opinion of this reviewer that a precipitate is not "technically" a nanoparticle. The authors have not dispersed nanoparticles into an Al-based alloy, rather produced these precipitates via conventional and wellknown metallurgical methods. I tend to view nanoparticles as nanomaterials that are not necessarily are embedded into a matrix. Some philosophical comments for the authors to think about! Precipitate coherency is mentioned in line 95, but no coherence study between the alloy and the precipitates has been performed. Similarly, the authors mention in lines 108-109 those mechanical properties are significantly improved "(…) due to the change in nanoprecipitates (…)", but no clarification has been provided. Micrographs in Fig. 2F and 2J are too small and of low resolution to detect any intergranular crack. Fig. 2 needs to pass a complete review and redesign.
When the authors say "(…) at the same hydrogen content level (…)", this reviewer thinks: how precise are the existing experimental methods for hydrogen detection in metals? Are they so precise one can claim levels are the same? Fig. 3 is clear and good. Increase font size will help Nature Communication's readership to better grasp the figure's message. On Fig. 4, plots 4E and 4F, I miss some error bars and a critical evaluation of the systematic and statistical errors that may arise during the measurement of hydrogen concentrations. This is of paramount importance to substantiate the author's nice discovery! I would like to give another opportunity for the authors to streamline all the discussions presented between lines 135-236. By streamlining this review means that the authors should: (i) stick with their discovery; (ii) answer clearer why T-phase precipitates are better than eta-prime (and others) to remove hydrogen interstitials from the alloy's matrix; and (iii) the implications of their discovery faced by previous literature.
One point is missing in the authors discussion though. Recently, it has been reported that T-phase precipitates are able to resist impact to energetic particle irradiation in a novel aluminium crossover alloy (https://doi.org/10.1002/advs.202002397). It has been reported that the T-phase is able to survive higher doses than other hardening phases in commercial aluminium alloys (i.e. Mg2Si in 6xxx series Albased alloys). One of the explanations provided by the authors to explain such a radiation tolerance is the fact that in aluminium crossover alloys, the volumetric fraction of T-phase precipitates is significantly higher than other phases in commercial aluminium alloys. If the number of precipitates divided by the volume is higher, then they act as a powerful sink for irradiation-induced defects. This reviewer question if the same thing happens here in this discovery reported by Y. Wang et al. in this present submission? Perhaps, the enhanced resistance to hydrogen embrittlement arises since upon precipitation of T-phase precipitates, the precipitation density increases considerably compared to eta-prime, leading to higher sinking efficiency for hydrogen.

Dear reviewers
We appreciate all the efforts spent in reviewing our manuscript and are really thankful for the constructive comments. As required by the reviewers, we performed additional experiments and analyses to further improve the quality of this manuscript.
The point-by-point response is listed as follows. All the changes were marked in yellow background in the revised manuscript.

Reviewer #1 (Remarks to the Author):
The issue of HE in high strength materials has been a topic of interests for centuries now; and this work is no exception.
I commend the authors for this comprehensive work on the HE of Al. The research is original, and your combination of simulation and experimental work is deserving praise as it is well-thought out. There was a conscientious effort to present results effectively and provide convincing evidences to arguments. Data analysis and interpretation is logical; and the conciseness of the work is also meritorious.
Your work presents a significant finding related to HE mitigation and will be useful in terms of providing insights to future mitigation techniques for other metals.
Please see attached for some minor comments and suggestions to help improve the quality of the work.
Response: We really appreciate the recognition from the reviewer and are glad to hear that we are on the right track.
Comment: I am a bit concerned with the corrugated shape of the stress-strain curve in the plastic region (Fig. 1A). Can you explain what caused this?
Response: As explained in the Materials and Methods in supplementary materials, quasi in-situ tensile tests were performed in the present study to obtain a series of 3D images of the Al alloys. In this sort of interrupted tensile tests, the specimen was hold still for a period of time (30 mins in present study), during which thousands of CT images were acquired.
The corrugated shape of stress-strain curve is due to the stress relaxation that naturally occurs during specimen holding. The interrupted tensile test has been widely utilized to study 4D evolution behavior of microstructures within materials, by combining it with X-ray However, there are indeed some disadvantages of this interrupted tests although they are popular and useful in the 4D observations, possibly including those issues that concern the reviewer. One is that it becomes tricky to get the accurate fracture strain from the serrated stress-strain curves due to the small size of specimen (no strain gauge was used for such small specimen). To solve this problem, the true strains in each step were corrected by precisely measuring the displacements of different pairs of particles within the specimen during plastic deformation. The strain values between different two neighboring steps were calculated by linear interpolation. The fracture strain was predicted by linear extrapolation based on the two strain values in preceding two steps. Therefore, the corrugated shape does not affect the measurement of the elongation loss, which has been used as an indicator of hydrogen embrittlement.  Response: We agree that the techniques used for obtaining the 3D strain maps in Fig. 3 should be described more clearly in the text. The high-density strain maps were generated by tracking the movement of numerous particles at different loading steps within the specimen, using the MATLAB-based image segmentation and tracking algorithms developed in the authors' lab, i.e., the so-called microstructural feature tracking (MFT) technique.
In short, the same particles (in this case the micron-scale Al2MgCu particles) at different loading steps were precisely matched by comparing their gravity centers, volumes and surface areas. This enables the generation of high-density 3D strain maps, by dividing the specimen interior into numerous tetrahedrons with the tracked Al2MgCu particles (with a total number of 55, 890) as vertices.
The following steps are necessary in the mapping of 3D strains: 1. Quantitative geometric analysis. Polygonization techniques such as the marching cubes algorithm introduced serve as the basis of 3D image analysis. Volume, surface area, and length measurements can be accurately conducted once the surface morphology has been determined.
The MATLAB-based home-made software used by the authors is capable of analyzing various microstructure parameters, including those used for measuring the size, shape, and spatial distribution.
2. Particle tracking. First, an affine transformation was applied to register the 3D images in two loading steps, which is the key to the success of following tracking analysis. The matching probability parameter (Mp) was defined to evaluate the matching accuracy of particles before and after deformation, by comparing the gravity center, volume and surface area of these two particles, and a threshold Mp value was applied to filter out the mis-matched particles.
3. Strain mapping. The sample interior can be divided into multiple tetrahedrons with the tracked particles as vertices by using the Delaunay tessellation. Assuming the deformation is small, the vertical strain (εx, εy, εz) and shear strain (γ xy, γ yz, γ zx) in a single tetrahedron can be calculated from the particle deformation (ui, vi, wi), (uj, vj, wj), ( uk, vk, wk), and ( ul, vl, wl) at four vertices i, j, k, and l.
This process can be illustrated in Fig. R1 shown below ('X-ray CT', Hiroyuki Toda, Springer, 2022, Chapter 9. 4D image analysis, Fig. 9.3): In the calculation of 3D strain maps, the quantitative data (so-called object data) including the X/Y/Z coordinates of gravity center, equivalent diameter and sphericity of Al2MgCu particles in text files were imported into the program. Then the strain maps were automatically generated without the necessity of visualizing the movement of particles, although tracking the particle movement is the principle of the strain mapping technique.

Reviewer #2 (Remarks to the Author):
The manuscript report the T phase based Al alloy to enhance HE resistance. I have a few questions below 1. The authors claim the ability of T phase to H trapping from DFT simulations.
It will be more convincing to compare such kind of results with other precipitates for example MgZn2. Moreover, the authors claim no need to include zpe. More evidence is needed. More over, the entropy will also be involved in addition to zpe, and how the binding energy change with the H concentration is also of interests to see the real capacity to trap H.
Response: We agree that the comparison of hydrogen trapping at T phase with other precipitates is important to understand the main discovery in the present study. Thirdly, the change of trap energy with multiple hydrogen atoms has also been studied, as shown in Fig. R3 below, which indicates that T phase with 24 crystallographically equivalent trap sites in its unit cell is capable of trapping a considerable mount of hydrogen atoms. In the revised manuscript, these points were emphasized. See main text in Page 8 and Supplementary Table 1: 'Moreover, spontaneous debonding at the coherent interfaces of the η phase (with a maximum hydrogen binding energy of 0.3 eV) may also occur…' 2. How about T phase compared to other precipitates other than the η' phase? Response: Hydrogen trapping energies at numerous types of particles were analyzed in the authors' lab, as shown in the figure

Figure R4 Summary of trapping energies at various intermetallic particles
These particles include the common precipitates in Al alloys, such as Mg2Si, Al7Cu2Fe, Al2MgCu (S phase in the present manuscript) and so on. For some of these precipitates, hydrogen is trapped at interfaces, such as Mg2Si and MgZn2; for others, hydrogen can be trapped at the interior, such as Al2MgCu, Al7Cu2Fe, and Al11Mn3Zn2. Hydrogen trapping energy of T phase is higher than most of these particles. The Al7Cu2Fe, and Al11Mn3Zn2 particles with very high binding energy are believed to be similar with or more effective in hydrogen trapping than T phase, but due to their large size (which causes significant stress localization and thereby likely defect initiation) they are considered less effective in suppressing hydrogen assisted crack growth than T phase. In conclusion, we consider T phase as the best candidate among these precipitates for hydrogen embrittlement suppression.
These issues including have been briefly discussed in the previous manuscript. See  Supplementary Fig. 4 below.
Supplementary Fig. 4. HAADF-STEM image of the interfaces at MgZn2 precipitates in HT material, indicating semi-coherent interfaces at the side surfaces.
Coherent η/Al interfaces trap H with a relative lower binding energy (0.3 eV), but H atoms strongly decreases the binding energy until zero, i.e., the hydrogen induced spontaneous deboning at coherent η/Al interface reported by Tsuru et al.
The 3D interior of T phase traps hydrogen with a high binding energy (with a maximum of 0.6 eV), and they trap more H than 2D semi-coherent η/Al interfaces. Even in the presence of some fractions of semi-coherent η/Al interfaces in HT material, T phase is still effective in H trapping. Specifically, as shown in the revised Fig. 4e, hydrogen concentration at T phase is one order of magnitude higher than that at semi-coherent η/Al interfaces.
In the first-edition manuscript, all the interfaces at MgZn2 were assumed to be coherent.
In the revised one, all the side interfaces at the MgZn2 in HT material were assumed to be semi-coherent. This is a conservative assumption for estimating the trapping effect of T phase, keeping in mind that semi-coherent interfaces at MgZn2 trap more hydrogen than coherent ones because of their high binding energy (0.56 eV), and the detrimental effect of hydrogen-assisted interfacial debonding at MgZn2. In other words, using this assumption, we estimate the lowest amount of hydrogen trapped by T phase, in the competitive hydrogen trapping scenario between MgZn2 and T phase.
In terms of strain's effect, it is believed that the spherical-shaped T phase exhibits much lower stress localization than that at the edges of elongated MgZn2. Therefore, it is reasonable to assume that the T phase suppresses hydrogen embrittlement by reducing the risks of crack initiation around MgZn2 phase, which is expected to be strongly related to the stress/strain distributions at its edges.
More discussions about these issues have been added into the revised manuscript.
4. What is the general guidance to the design HE resistance alloy? It will be better the authors could elaborate on that. interfaces, thereby high trap site density); round shape and small size endow them with low stress localization. These issues are more clearly summarized in the conclusion part.

Reviewer #3 (Remarks to the Author):
This paper is extremely interesting.
In fact, I did not see any mention in the manuscript, but the AlMgZn(Cu) alloys, which is the metallurgical merge between 5xxx and 7xxx series of Al-based alloys, comprise an entire new class of alloys, which has been discovered just in the past 3 years: the aluminium crossover alloys. A recent review paper has been published on the topic and I suggest the authors to read it and possibly incorporate these ideas into their manuscript, if they find it appropriate and suitable (https://doi.org/10.1016/j.pmatsci.2021.100873).

Response:
We appreciate the constructive comment. Although the alloy class in the suggested paper (crossover alloys with low Zn/Mg ratios) is different from ours (7XXX, with a Zn/Mg ratio > 1), we found some ideas are interesting in the paper and they can be incorporated into our manuscript.
This reference pointed out that these crossover alloys exhibit strong potential to achieve better trade off between strength and other properties, i.e., formability in their case. This is well aligned with the findings in our study, i.e., T phase shows good potential to overcome strength-HE conflict. From this standpoint, the crossover alloys have attracted increasing more attention. Such trend has been added into the discussions in the revised manuscript. See the last paragraph of main text.
Interestingly, one of the major features of novel aluminium crossover alloys is the precipitation of the T-phase -Mg32(Zn,Al)49 -which is a highly chemically-complex intermetallic phase and highly-concentrated superstructure bearing hundreds of atoms in its unit cell; this is opposed to most conventional hardening precipitates (and not nanoparticles as the terminology used by the authors in their paper) in existing commercial aluminium alloys.
That said, I will now make some comments on the authors' paper: this is a very good piece   However, as shown in the above Fig. R5, the characteristic diffraction spots for T and η phases are very close to each other. Moreover, in the present study, the diffraction spots for T phases are too weak. So, we preferred SAED data along [110] zone axis, to clearly present the characteristic spots for T phase, without strong interference from η. Comment: In addition, for an article of the Nature Communication's relevance, this reviewer finds it mandatory that the authors present some sort of chemical analysis of these precipitates: either APT or STEM-EDX. These phases can be very similar on a chemical nature standpoint, and unequivocal confirmations of their existence are mandatory for validate the main message of the paper, which is the hypothesis that T-phase precipitates contribute to a significant reduction in the concentration of hydrogen within an Al-based alloy (a new crossover alloy type synthesized by the authors?). In some aluminium crossover alloys, the diffraction signal of T+eta-prime phases come together, but there are ways to distinguish them; I advise the authors to consult appropriate literature.

Response:
We found it is difficult to accurately measure the chemical compositions of both η and T phases using STEM-EDX, due to the small size of these two precipitates. As APT results indicate that the total atomic percentages of Zn+Mg in many precipitates are fairly high (close to 80%), as shown in Fig. 1h. This is an evidence of T phase (Zou et al., Mater. Character., 2020, https://doi.org/10.1016/j.matchar.2020.110610 ). In contrast, the atomic percentage of Zn+Mg in η phase is relatively lower, around 60% (Fig. 1i).
Comment: Now a comment on semantics. Can a nanoparticle be considered a precipitate and vice-versa? Well, precipitates (i.e. a secondary phase that separate/partition out the alloy's matrix whilst still embedded to it) via natural and artificial aging are known to occur in aluminium alloys since 1938 (Nature 142, 569-570 and Nature 142, 570-570 both from the year of 1938!). The term "nanoparticles" gained only recent attention due to the emergence of nanotechnology as a consolidated field of research. Personal opinions in paper revisions can cause stress to the authors, but it is the subjective opinion of this reviewer that a precipitate is not "technically" a nanoparticle. The authors have not dispersed nanoparticles into an Al-based alloy, rather produced these precipitates via conventional and well-known metallurgical methods. I tend to view nanoparticles as nanomaterials that are not necessarily are embedded into a matrix. Some philosophical comments for the authors to think about!

Response:
The review proposed a really good comment about the terminology. Initially the authors changed 'precipitates' to 'particles' to fit the wide readership of this journal. Now we have decided to change back to 'precipitates'. We think the precipitates that form during quenching and aging treatment can be considered as particles, whereas the intermetallic compound particles are not necessarily precipitates (such as those particles that are purposefully added into the matrix). Although in the present study, all the second phase particles including T, η and S phases can be defined as precipitates, for clarity consideration, we refer to the nano-sized T and η phases as 'precipitates', whereas the coarse S phase as 'particles'. In this way, the 'precipitate switching' strategy proposed in the present study is only related to T and η phases. Semi-coherent interfaces were found for MgZn2 phase in HT material. Therefore, the assumptions were modified in the hydrogen partitioning analysis (fully coherent interfaces were assumed in previous manuscript) and accordingly Fig. 4e and f were updated. The revised partitioning analysis indicates that some fractions of hydrogen go to semi-coherent interfaces due to their high binding energy. But this does not affect the trapping effect of T phase, because of their large trap site density (3D interior in T phase compared to the 2D interfaces at η phase.
2. The T phase leads to the improvement in ductility in the stress-strain curve in Fig.   2a and reduces the areal fractions of intergranular cracks on the fracture surfaces. In the revised manuscript, the 'improved mechanical properties' was stated more clearly.
3. Two magnified SEM images of IGC were added to Supplementary Fig. 6.
Comment: When the authors say "(…) at the same hydrogen content level (…)", this reviewer thinks: how precise are the existing experimental methods for hydrogen detection in metals?
Are they so precise one can claim levels are the same?
Response: The thermal desorption analysis utilized in the present study is a very precise technique, that is able to measure hydrogen content in ppb level. The hydrogen concentrations for LT and HT specimens are 774 and 783 ppb (as shown in the TDA curve in revised Fig. 2b), respectively. It is thereby believed that this method is accurately enough to ensure same hydrogen levels within the specimens. Response: We thank the reviewer for this suggestion. The font size was increased. The formats of all the figures were carefully checked to improve the quality.
On Fig. 4  Response: Thanks for the suggestion. We agree that the discussions provided in previous manuscript were to some extent deviated from the main logic line. This part was revised according to this comment, while at the same time the conciseness was maintained.
1. Some discussions were added to connect different paragraphs and to make sure all these parts are close related to the main topic of this manuscript. Some unrelated discussions were deleted: 'Instead, particle damage occurs naturally in the form of internal breakage in a similar way to that in ductile fracture 39 , except that the void growth is accelerated in the plastic zone due to the large strain level and high stress triaxiality.' 'As suggested in theoretical models, a peak hydrostatic stress appears at some point in front of a crack tip, where GB decohesion is likely to occur first, under the influence of stress-enhanced diffusion toward the stressed region due to lattice expansion' 'In contrast, the hydrogen microvoids originating from the breakage of particles, due to their high binding energy, can resist crack growth to some extent by trapping a considerable amount of hydrogen inside them as hydrogen gas 24 ' 2. The role of T phase is more clearly stated, including its hydrogen trapping features (i.e., the high binding energy and large volume fraction), low stress localization (i.e., spherical shape, small size), and they are in depth compared with other precipitates.
One point is missing in the authors discussion though. Recently, it has been reported that T-phase precipitates are able to resist impact to energetic to elevate the aging temperature from 120 to 150 degC. As shown in the SAED patterns in Fig. 1b and Fig. 1d, the characteristic spots for T phase appears in HT material. This means the volume fraction of T in HT material has been successfully increased, leading to higher sinking efficiency for hydrogen, as suggested by the reviewer.
Moreover, other than volume fraction (trap density) of T phase, it is believed that the hydrogen trapping features of T phase play even more important roles. On one hand, compared to the hydrogen trapping at 2D interfaces at MgZn2, T phase strongly traps hydrogen in its interior (3D space), which means single T phase particle is capable of trapping more hydrogen atoms; on the other hand, the hydrogen trap energy at T phase is much higher than that of coherent MgZn2 interfaces, which endows T phase with strong hydrogen trapping capacity, because hydrogen trapping is strongly related to the binding energy at trap sites, as illustrated by the following Fig. R8:   Fig. R8 dependence of hydrogen occupancy at trap sites on their hydrogen trap energies It is seen from Fig. R8 that when the trap energy is elevated by 0.1 eV, hydrogen occupancy can be increased by around 2 orders of magnitude.
More discussions regarding these aspects are provided in the revised manuscript.
In conclusion, we have tried our best to improve the quality of this manuscript and all the comments from three reviewers have been addressed.
We hope the revised version is acceptable for publication.
We would like to thank the reviewers again for your time and consideration.
Best wishes.